1. Field
The following description relates to systems and methods utilizing a measure of neutron energy as a filter to determine the Plutonium and/or fissile material content of nuclear material such as mixed-oxide (MOX) reactor rods.
2. Description of Related Art
The detection of neutrons as a probe of material structure and composition has been in use over many decades. Using charged radioactive particles as probes is straightforward as they leave a path of disruption in the material through which they pass which disruption can be measured with ease. The challenge in detecting neutrons is that they are neutral particles resulting in no path of disruption. Detection of neutrons requires that the neutron interact with the nucleus of an atom in the material which it is transiting. The usual result of such an interaction is the emission or recoil of a charged radioactive particle which is subsequently detected. The likelihood of such an interaction can vary by many orders of magnitude depending on the material involved.
Most commonly the material used for these detections is an isotope of Helium, He-3, which has a relatively large likelihood of an interaction with neutrons of very low energies often referred to as thermal neutrons. The capture of the thermal neutron on He-3 results in the emission of a tritium nucleus and a proton both of which can be easily detected. The He-3 is used as a fill gas for a proportional counter which detects the resulting emissions of thermal neutron capture.
Other materials in use for neutron detection are based on the large likelihood of a thermal neutron interaction with an isotope of Boron, B-10. Detectors of this type are filled with boron-trifluoride gas (BF3), or have walls of the proportional counter coated with B-10. U.S. Pat. No. 2,443,731 (which is incorporated herein in its entirety by reference) provides an early citation of these approaches.
All these detectors measure only thermal neutrons. However, many applications result in neutrons of high energy, sometimes referred to as fast neutrons. To be detected by these detectors requires that the energy of these fast neutrons be lowered or thermalized using elastic collisions in hydrogenous materials such as polyethylene.
There is a natural background of neutrons resulting from cosmic ray interactions in the atmosphere and other sources. To be detectable the flux of neutrons coming from the source of interest must be generally equal to or greater than this background. Other measurement methods, such as coincidence measurements, can also make the neutrons of interest more detectable. In coincidence measurements two or more neutrons are emitted simultaneously by the object of interest. These neutrons are detected together within a very short interval.
A unique feature of the fissile radioactive material, notably Plutonium (Pu), is that when it spontaneously fissions it emits several neutrons at the same time. The energy spectrum of these emitted neutrons is often referred to as a Watt spectrum named after B. E. Watt, a Los Alamos scientist who measured the spectrum for the first time in 1952. In addition to the natural background there are other reactions such as the alpha particle interactions with Oxygen which can produce neutrons sometimes referred to as (alpha,n) reactions. This source of neutrons can be significant as alpha particles are very commonly emitted by fissile materials. When these fissile materials are embedded in an Oxygen compound such as in a MOX material, neutrons from (alpha,n) reactions can be very likely. Discriminating between Watt spectrum neutrons and (alpha,n) neutrons can provide a measurement of the matrix in which the fissile material is embedded. However, these neutrons are emitted singly and not in simultaneous multiples. This allows for the use of coincidence measurement to determine the presence of Pu isotopes and other fissile materials in reactor rods. U.S. Pat. No. 3,222,521 (which is incorporated herein in its entirety by reference) is an early patent citation using this multiplicity approach to determine the fissile content of reactor rods.
In addition to the background neutrons and the neutrons not of direct relevance to the presence of fissile material, there is a significant flux of gamma rays coming from natural sources and from the radioactive materials of interest and the matrix in which they are embedded. To be usefully used in these applications, neutron detectors need to not only be capable of detecting neutrons but also be relatively insensitive to gamma rays or have a method by which the gamma ray and neutron signals can be separated electronically. The He-3, BF3, and B-10 based detectors are very insensitive to gamma rays, and are incapable of delivering energy information.
To accomplish the coincidence measurements requires electronic circuitry that can measure that a signal coming from one of the detectors is above a set or predetermined amplitude and the time of the detection of the signal. There is also needed a program on a computer for analyzing the data or more generally a filter that requires two or more signals be detected with an amplitude sufficient to be indicative of a neutron and that these signals be detected within a narrow time interval. U.S. Pat. No. 3,456,113 (which is incorporated herein in its entirety by reference) is an early patent citation describing this type of circuitry and analysis system.
The above discussed measurement approaches have been further and extensively discussed in many other basic research literatures and in U.S. Pat. Nos. 2,741,705; 2,842,695; 2,920,204; 2,830,185; 4,201,912; 4,483,816; 4,617,466; 4,920,271; 5,197,130; 6,341,150; 6,420,712; and 6,509,563; the entire contents of all of which are incorporated herein by reference.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.